It could be a lot of things.
The Rams may not have given Garrett everything he wanted. They may not have paid him what he thought he deserved. He may have thought the job would be a mess with a franchise that might want to move in a year or two. He might have decided that working in Dallas isn't all that bad.
Or the Rams might have changed their minds. They might have seen some of their fan reponse and decided to go in another direction. They might have had nightmares of Mike Martz and Scott Linehan.
Any number of things could have happened. Garrett didn't get the job in St. Louis, and it looks like he is your Dallas Cowboys coordinator again this season. Now, what does that mean for the fans, the players, and management in Dallas? That's my biggest issue.